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Breast cancer and smoking: What is the link?

Karis Betts
by Karis Betts | Analysis

9 July 2024

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A patient preparing for a breast screening (mammogram) with a healthcare professional
Credit - Patrick Harrison

After years of growing evidence and in agreement with other organisations, Cancer Research UK have included breast cancer in its analysis of smoking and cancer risk. Here, we take a closer look at what that means.  

Cancer is not a simple disease. A person’s risk of cancer can be affected by many things. These are known as risk factors and include things we can’t change, like getting older and our genetics, but also risk factors that we are exposed to in our environment like smoking and alcohol.  

Some cancers are caused by a complex mix of risk factors, but sometimes cancer also develops by chance.  

And different types of cancer have different risk factors that can cause it, and in varying degrees.  

At Cancer Research UK we regularly review published evidence and scientific opinion about cancer risk factors.  

These reviews consider both evidence strength, how confident we are that the risk factor definitely causes cancer, and effect size, how much of an impact the risk factor has on the chance of developing cancer.  

And we use the results from those reviews, to create our own figures on the total number of cancer cases attributable to different risk factors in the UK.

Smoking – the biggest risk factor for cancer 

We already know that smoking is the biggest cause of cancer in the UK and the world.  

And even though smoking rates are falling, new analysis has shown that cases of cancer caused by smoking in the UK in 2023 have reached an all-time high 

It’s estimated that smoking causes around 57,600 cases of cancer in the UK every year, that’s around 160 cases every day. 

The obvious cancer type linked to smoking is lung cancer, with smoking causing more than 6 in 10 cases of lung cancer in the UK, or around 33,100 cases a year. But smoking doesn’t just cause lung cancer. It can actually cause 16 different types of cancer, including mouth, liver and bowel.  

Cancer Research UK’s most recent analysis of the burden of smoking, includes the link between tobacco and breast where previous iterations of the research have not. 

This doesn’t mean that smoking has only recently started causing breast cancer, but that it’s only recently that we’ve had strong enough scientific evidence to be sure. 

Breast cancer and smoking 

The potential link between smoking and breast cancer has been known for some time. People often think that science is full of big discoveries and ‘Eureka’ moments giving us the answer.  

But a lot of the time, we’re building the puzzle piece by piece and might have to wait years, even decades to build up the evidence to be certain of something. 

There are global bodies of experts whose role it is scrutinise scientific evidence and determine how confident we can be that exposure to a certain risk factor is a definite cause of a certain cancer type.  

They take into account a wide range of types of evidence, including studies of large groups of people followed up for decades to see how their environment and behaviour is associated with whether or not they get cancer later in life, and animal and human cell studies to understand the biological mechanisms by which a risk factor causes cancer. 

These include The International Agency for Research on Cancer (IARC), the specialized cancer agency of the World Health Organization (WHO), World Cancer Research Fund (WCRF) and Global Burden of Disease (GBD).  

But studying risk factors can be particularly tricky. This is because people have different exposures to a range of different risk factors through their environment and behaviours, so it can be very hard to find the effect of just one thing.  

And sometimes the biological mechanism of effect isn’t clear in those studies of animal or human cells. 

It can often take us years to get enough high-quality evidence to be certain of risk factors for cancer.  

For example, people who smoke may also drink alcohol, be overweight or have a family history of breast cancer, all things that can increase their risk of developing the disease. 

And cell studies have shown that though tobacco smoke contains cancer-causing chemicals and does reach the breast tissue, smoking is also linked with lower levels of the hormone oestrogen, which itself is linked with higher risk of breast cancer. 

What do these global bodies say? 

IARC publishes a series of reports, called Monographs, assessing whether something has the potential to cause cancer. IARC last published a Tobacco Smoke and Involuntary Smoking Monograph in 2004. Tobacco was included again in the Personal Habits and Indoor Combustions volume in 2012.  

At that time, they could only suggest a possible link between smoking and breast cancer based on the available evidence, reporting that there is limited evidence that tobacco smoking increases the risk of breast cancer.’ 

However, in the last 20 years the evidence has built up with better ways to control for confounding factors like alcohol, larger and longer-term studies able to look at the effect of smoking at different times of life and on different breast cancer subtypes, and more clarity on the biological mechanisms at play.  

Both WCRF and GBD now recognise tobacco as a risk factor for breast cancer following their expert evaluation of the scientific evidence.   

What does the risk mean?   

The risk that smoking confers to breast cancer is actually relatively small, with risk of the disease around 10% higher in women who currently smoke compared with those who have never smoked.  

This combined with falling smoking prevalence over the years, means tobacco is estimated to cause around 4% of breast cancers in the UK currently. That’s around 2,200 cases a year. This is much smaller than the around 33,100 lung cancers smoking causes each year.  

It’s also important to bear in mind that a risk statistic can’t tell you your individual risk of cancer.  

That’s because an individual’s risk of cancer is based on many different factors, including age, genetics, and the environment. But risk statistics can help us to learn more about how we can reduce our risk of cancer and they can tell policymakers how to prioritise interventions for different risk factors, to have the biggest impact on population health. 

You can find out more about the risks and causes of breast cancer on our breast cancer pages. 

Reducing your risk of cancer 

There are never any guarantees against cancer. But there are things you can do to reduce your risk of cancer, and making small, healthy changes in your life can really add up.  

Not smoking, keeping a healthy weight, and staying safe in the sun are all proven ways to lower your risk of cancer.  

If you want to find out more about how cancer can be prevented visit our health information pages on how you can lower your risk of cancer. 

If you want to find out more about risk factors for cancer and how you can lower your risk, visit our health information pages on the causes of cancer. 

Stopping smoking completely is the best thing you can do for your cancer risk, as well as your overall health.  It can be difficult, but it’s never too late to stop smoking. Take a look at our advice on how to stop smoking and get support from your free, local stop smoking service, doctor or pharmacist.  

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